Conventionally, chromium has been plated from aqueous chromic acid baths prepared from chromic oxide (CrO.sub.3) and sulphuric acid. Such baths, in which the chromium is in hexavalent form, are characterized by low current efficiency. The chromic acid fumes emitted as a result of hydrogen evolution also present a considerable health hazard. Furthermore, the concentration of chromium in such baths is extremely high, leading to problems of waste or recovery because of so-called "drag-out" of chromium compounds into the rinse tanks which follow the plating bath.
To overcome many of the disadvantages of hexavalent chromium plating, it has been proposed to plate chromium in trivalent form. One such process for plating chromium from an aqueous solution of a chromium (III)--thiocyanate complex is described in UK Pat. No. 1,431,639 and its equivalent U.S. Pat. No. 4,062,737. Another such process is described in out UK patent application No. 24734/77 and its equivalent U.S. Pat. No. 4,161,432 which describes a chromium plating solution and process in which an aqueous solution of a chromium (III)--thiocyanate complex is again employed but in which a buffer material supplies one of the ligands to the chromium complex. The buffer material is selected from amino acids (e.g., glycine, aspartic acid), peptides, formates, acetates and hypophosphites.
These trivalent chromium plating processes do not give off chromic acid fumes. They are of high efficiency with a wide plating range and good covering power. A very much lower amount of chromium is needed in the bath than is the case with hexavalent processes, thus reducing the problems associated with "drag-out". Concentrations of chromium have ranged from 0.30 to 0.5 Molar.
Although the trivalent chromium plating processes of UK Pat. 1,431,639 and UK patent application No. 24734/77 overcome all the major disadvantages of hexavalent plating, the appearance of the deposited chromium is generally somewhat darker. While this color is quite acceptable or even preferable for many applications, it is advantageous to be able to plate lighter colored chromium with a trivalent process. In UK patent application 44177/78, chromium (III)--thiocyanate baths are disclosed whose chromium concentration is far below the generally accepted level for efficiency and bath stability. Such baths give a significantly lighter colored deposit. The concentration of chromium in these baths is less than 0.03 M and preferably less than 0.015 M.